Najas marina aka Marine Naiad

Najas marina is a water plant in the Hydrocharitaceae family. Common names include spiny naiad, holly-leaved naiad, and pondweed. Its native range spans much of the world. The RHS lists it as native to Britain and Ireland. It grows in warm and temperate regions, but avoids colder areas. It lives in ponds, lakes, and rivers with slow flow. Both fresh water and brackish water suit it.

The whole plant grows beneath the water's surface. Its green leaves range from pale to bright shades. Sources give a height of about 1.6 feet [0.5 metres]. Stems can reach 2.6 feet [0.8 metres] in length. These stems snap with ease, and broken pieces can root. Those pieces can then grow into new plants. Sources differ on its life span. Kew classes it as an aquatic annual. Plants For A Future and Useful Tropical Plants call it perennial.

Male and female flowers grow on separate plants. Water carries the pollen, and fertilization takes place beneath the surface. Both sexes must grow together to produce seed. Plants For A Future gives September to November as flowering time. It gives the same months for seed ripening. The sources offer no further detail on the fruit. Division during the growing season is one listed way to propagate it. Its ability to root from broken stems offers another route.

For cultivation, this plant needs water rather than damp soil alone. It grows well in shallow fresh or brackish water. Wild plants can occur as deep as 19.7 feet [6 metres]. It accepts full sun or partial shade. Sand, loam, and clay all suit its roots. It tolerates mildly acid, neutral, and alkaline conditions, including saline sites. Growers use it in ponds and sometimes in aquariums. The evidence gives no firm frost-hardiness limit. Sources also disagree about its weed potential.

People gather wild plants for food, including for markets in Hawaii. Plants For A Future lists leaves as edible. It also records eating the young plant raw. Both use sources report no known hazards. That does not establish safety for every person or pet. Plants For A Future lists no known medicinal use. Useful Tropical Plants reports an IUCN Least Concern assessment from 2019. It describes populations as stable across most of the range.